1. Field of the Invention
The present invention relates to an ignition apparatus for internal combustion engine including an ignition sensing circuit for confirming whether ignition operation is normally carried out, and more specifically, to an ignition apparatus for internal combustion engine capable of easily and securely sensing an ignition state based on the voltage level of an ignition signal to a power transistor.
2. Description of the Related Art
Conventionally, ignition apparatuses for internal combustion engine electronically calculate and control an amount of fuel to be injected into each cylinder and an ignition timing using a microcomputer.
Further, when misfiring is caused by the faulty wiring and the like of an ignition system, fuel injection must be stopped by sensing misfiring in the ignition system so as to prevent the exhaust of uncombusted gas.
Although a method of sensing the primary voltage of an ignition coil and determining whether misfiring is caused or not is conventionally proposed as this type of an ignition sensing apparatus, it is difficult to correctly sense misfiring by such a method. A reason for it will be described with reference to FIGS. 15 (a) and 15 (b).
FIGS. 15 (a) and 15 (b) are respectively waveform diagrams showing the variation in time of the voltage level of a primary voltage, wherein FIG. 15 (a) shows a waveform in a normal state and FIG. 15 (b) shows a waveform when an abnormal state arises (for example, the voltage level drops to about one third that in the normal state due to faulty wiring and the like) respectively. In FIG. 15 (a), a solid line shows a waveform when the secondary side of an ignition coil is made open and a broken line shows a waveform when firing is carried out to an ignition plug connected to the secondary side, respectively.
In FIGS. 15 (a) and 15 (b), c denotes a trigger level as a comparison reference for determining whether firing is carried out normally or abnormally, and the trigger level c must determine that firing is carried out normally in the case of FIG. 15 (a) and that firing is carried out abnormally (i.e., misfired) in the case of FIG. 15 (b) according to the intrinsic object thereof.
However, since the initial value of a primary voltage is greater than the trigger level c in both the cases of FIGS. 15 (a) and 15 (b), they are determined to be in a normal firing state.
To cope with this problem, although there is proposed an ignition apparatus for internal combustion engine for determining whether misfiring is caused or not by sensing a primary current as shown for example, in Japanese Patent Publication No. 6-60626, there is a problem in the apparatus that a current sensing resistor must be provided to the passage of the primary current and thus the number of parts and circuits is increased.
As described above, the conventional ignition apparatuses for internal combustion engine have a problem that they cannot correctly determine an abnormal state when they determine the abnormal state of an ignition system based on the primary voltage of an ignition coil because even if a primary current drops to about one third that in a normal state by any reason, the primary voltage is not different from that in a normal operation.
Further, when the abnormal state of an ignition system is determined based on the primary current, a problem arises in that the number of parts is increased and thus a cost is increased because a current sensing resistor must be provided and a current sensing circuit must be connected to the resistor.